1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly to variable microstrip, buried microstrip and stripline filters.
2. Description of the Related Art
A filter is a frequency-selective signal transmission device in which certain ranges of frequencies (the passband) are passed from an input to an output, while other ranges (the stopband) are rejected. Filters can be formed in many different ways. For example, one configuration, known as microstrip, places conductive traces (filter elements) on a board (substrate) surface and provides a second conductive layer, commonly referred to as a ground plane. Microstrip filter elements are each designed to have a specific impedance and/or signal response, which are determined by the trace geometry and the dielectric properties of the substrate material. Further, the conductive traces are arranged on the substrate in accordance with a selected filter topology. A second configuration, known as buried microstrip, is similar to microstrip except that the filter elements are covered with a dielectric substrate material. In a third configuration, known as stripline, the filter elements sandwiched within substrate between two electrically conductive (ground) planes. In all cases, the characteristics of the filter are determined in part by the electrical properties of the material (e.g. substrate) in which the conductive elements of the filter are embedded.
Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or ∈r) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is approximately inversely proportional to √{square root over (μ∈)}, and therefore affect the electrical length of a filter element. Further, ignoring loss, the characteristic impedance of a filter element, such as stripline or microstrip, is equal to √{square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Cl are generally determined by the permittivity and the permeability of the dielectric material(s) used to separate the filter elements from other transmission line structures as well as the physical geometry and spacing of the filter elements and transmission line structures.
In a conventional RF design, a substrate-material is selected that has a relative permittivity value suitable for the design. Notably, conventional substrate materials typically have a relative permeability of approximately 1.0. Once the substrate material is selected, the filter response is exclusively adjusted by controlling the topology of the filter and the geometry and physical structure of the filter elements.
One problem encountered when designing such filters is that the filters are generally optimized only for a pre-determined passband and stopband at a pre-determined impedance. If the filter is designed to have a wide passband to pass multiple signals at different frequencies, a greater amount of noise and undesired signals that happen to be in the filter's passband also will be propagated through the filter. On the other hand, if the filter is designed to have a narrow passband which limits the amount of noise and undesired signals that pass through the filter, only a limited range of desired signals will then be able pass through the filter. Modern RF circuits, however, commonly process multiple signals operating on different frequencies. An approach to address this dilemma is to make frequency selective properties of the filter variable. State of the art approaches to making the frequency selective properties variable generally include the use of mechanical means to alter the arrangement of the conducting elements of the filter, introducing a nonlinear component, such as a variactor, or digitizing the signal and implementing the frequency selection by numerical processing. Some approaches also vary the position or size of a dielectric component, for example a ferromagnetic inductor core whose position relative to inductor coil windings is varied by a screw mechanism, or a piezo-crystal whose dimension is varied in the presence of an electric field. However, such approaches provide only a limited range of adjustment for the frequency selective properties of the filter.